The present invention relates to a displacement sensing method and a motor control apparatus, more particularly to a displacement sensing method that corrects a measured angle and a motor control apparatus that corrects a measured angle to improve measuring accuracy.
A low output electric motor, which is called a galvano motor, is used for a processing machine such as a laser drilling machine, a laser trimming machine, and a laser repair machine. The galvano motor is required to have an angle sensor with high accuracy. Therefore, an incremental encoder is employed as an angle sensor of the galvano motor.
An optical rotary encoder and linear encoder are known as an incremental encoder. Such encoders sense displacement information such as a displacement amount and a displacement direction of an object to be sensed by using two signals (sine wave signal and cosine wave signal) which have a phase difference of 90° each other.
A lot of conventional encoders corrected amplitude of an output signal, an offset, and a phase to improve measuring accuracy. 
For example, Japanese Patent Laid-Open No. 8-145719 describes that an amplitude correction, an offset correction, and a phase correction are performed based on information on the maximum and minimum value of the output signal of the encoder and on the intersection of two-phase output signals. Japanese Patent Laid-Open No. 10-254549 describes that the amplitude correction and the offset correction are performed based on an average value of the maximum values and an average value of the minimum values of the output signal of the encoder.
However, conventional correction means performed the correction on the premise that the output signal of the encoder is an ideal sine wave signal. However, actually, since the output signal of the encoder includes a higher harmonic component or a nonlinear component, it is not an ideal sine wave. Accordingly, even if the conventional correction means correct the output signal of the encoder, the output signal of the encoder is not ideal sine wave signal.
A lot of encoders generate a plurality of split pulses including a phase difference corresponding to a split unit from the sine wave signal and the cosine wave signal in order to improve the sensing resolution of the displacement information of the object to be sensed. Such a method is called an electric split. The fact that the output signal of the encoder is not an ideal sine wave signal caused an error  in performing the electric split.
The encoder is processed so that the scale pitches have equal intervals. However, actually, it has a process error.